What kind of units does the Cycles emission strength use?
The unit is Irradiance - radiant flux emitted by a surface per unit area (watt per square meter).

This is a radiometric "corresponding" unit to Illuminance (also known as Lux), which is a photometric unit measured in lumen per square meter.

Radiometric units are based on physical power, that means all wavelengths are weighted equally, while photometric units take into account the sensitivity of human eye to different wavelengths. The weighting is determined by the luminosity function (which was measured for human eye and is an agreed-upon standard).

Converting Irradiance and Illuminance:

There is a different conversion factor for every wavelength, so the spectral composition of light must be known to make the conversion.

At the most sensitive wavelength to the human eye the conversion factor is

1.0 W/m2 = 683.002 lumen/m2  # at wavelength = 555nm (green)


That means the irradiance (power) to make 1 lumen is at it's minimum at this wavelength (just 1.464 mW/m2).

Luminous efficiency is then the ratio between the actual number of lumens per watt and the theoretical maximum.

Incandescent light bulb has a luminous efficiency of 2% which is very poor. It's because lot of it's irradiance is only heat which is not visible. The luminosity function is zero for wavelengths outside the visible spectrum.